Some years ago we described variants of B-cell lymphoma with a follicular growth pattern in children. The genetic basis of this lesion and how it differs from follicular lymphomas in adults has been a significant question. We showed recurrent genetic alterations of potential importance for its pathogenesis that disrupt pathways associated with the germinal center reaction (TNFRSF14, IRF8), immune escape (TNFRSF14), and anti-apoptosis (MAP2K1) have been described. In an attempt to shed more light onto the pathogenesis of PTFL, an integrative analysis of these mutations was undertaken in a large cohort of 43 cases previously characterized by targeted next-generation sequencing and copy number array. Mutations in MAP2K1 were found in 49% (20/41) of the cases, second in frequency to TNFRSF14 alterations (22/41; 54%), and all together were present in 81% of the cases. Immunohistochemical analysis of the MAP2K1 downstream target extracellular signal-regulated kinase demonstrated its phosphorylation in the evaluable cases and revealed a good correlation with the allelic frequency of the MAP2K1 mutation. The IRF8 p.K66R mutation was present in 15% (6/39) of the cases and was concomitant with TNFRSF14 mutations in 4 cases. This hot spot seems to be highly characteristic for PTFL. In conclusion, TNFRSF14 and MAP2K1 mutations are the most frequent genetic alterations found in PTFL and occur independently in most cases, suggesting that both mutations might play an important role in PTFL lymphomagenesis. Peripheral T-cell lymphoma (PTCL) is a group of complex clinicopathological entities, often associated with an aggressive clinical course. Angioimmunoblastic T-cell lymphoma (AITL) and PTCL-not otherwise specified (PTCL-NOS) are the two most frequent categories accounting for more than 50% of PTCLs. Gene expression profiling (GEP) defined molecular signatures for AITL and delineated biological and prognostic subgroups within PTCL-NOS (PTCL-GATA3 and PTCL-TBX21). Genomic copy number (CN) analysis and targeted sequencing of these molecular subgroups revealed unique CN abnormalities (CNAs) and oncogenic pathways, indicating distinct oncogenic evolution. Overall, our studies have demonstrated that novel GEP-defined PTCL subgroups likely evolve by distinct genetic pathways and provided biological rationale for therapies that may be investigated in future clinical trials. In other work we have studied novel cutaneous EBV-associated lymphoproliferative disorders. Patients with classic hydroa vacciniforme-like lymphoproliferative disorder (HVLPD) typically have high levels of Epstein-Barr virus (EBV) DNA in T cells and/or natural killer (NK) cells in blood and skin lesions induced by sun exposure that are infiltrated with EBV-infected lymphocytes. HVLPD is very rare in the United States and Europe but more common in Asia and South America. The disease can progress to a systemic form that may result in fatal lymphoma. We reported our 11-year experience with 16 HVLPD patients from the United States and England and found that whites were less likely to develop systemic EBV disease (1/10) than nonwhites (5/6). All (10/10) of the white patients were generally in good health at last follow-up, while two-thirds (4/6) of the nonwhite patients required hematopoietic stem cell transplantation. Nonwhite patients had later age of onset of HVLPD than white patients (median age, 8 vs 5 years) and higher levels of EBV DNA identified in peripheral blood. Our studies indicate significant differences in hydroa vacciniforme in patients from different ethnic/racial cohorts, indicating that genetic factors influence the severity of EBV infection in vivo. White patients with HVLPD were less likely to have systemic disease with EBV and have a much better prognosis than nonwhite patients. The WHO Classification of Tumours of Haematopoietic and Lymphoid Tissue notes instances of Burkitt lymphoma/leukemia (BL) with IG-MYC rearrangement displaying a B-cell precursor immunophenotype (termed herein preBLL). To characterize the molecular pathogenesis of preBLL, we investigated 13 preBLL cases using genome, exome, and targeted sequencing, imbalance mapping, and DNA methylation profiling. In 5 patients with reads across the IG-MYC breakpoint junctions, we found evidence that the translocation derived from an aberrant VDJ recombination, as is typical for IG translocations arising in B-cell precursors. Genomic changes like biallelic IGH translocations or VDJ rearrangements combined with translocation into the VDJ region on the second allele, potentially preventing expression of a productive immunoglobulin, were detected in 6 of 13 cases. We did not detect mutations in genes frequently altered in BL, but instead found activating NRAS and/or KRAS mutations in 7 of 12 preBLLs. Gains on 1q, recurrent in BL and preB lymphoblastic leukemia/lymphoma (pB-ALL/LBL), were detected in 7 of 12 preBLLs. DNA methylation profiling showed preBLL to cluster with precursor B cells and pB-ALL/LBL, but apart from BL. We conclude that preBLL genetically and epigenetically resembles pB-ALL/LBL rather than BL. Therefore, we propose that preBLL be considered as a pB-ALL/LBL with recurrent genetic abnormalities. In other recent work with studied cases of a novel form of aggressive lymphoma that closely resembles Burkitt lymphoma. Burkitt-like lymphoma with 11q aberration is characterized by pathological features and gene expression profile resembling Burkitt lymphoma but lack MYC rearrangement and carries an 11q-arm aberration with proximal gains and telomeric losses. Whether these lymphomas are a distinct category or a particular variant of other recognized entities is controversial. To improve the understanding of Burkitt-like lymphoma with 11q aberration we performed an analysis of copy number alterations and targeted sequencing in a group of cases. We showed that ID3, TCF3, or CCND3 mutations were absent in all cases, but other aberrations found in aggressive germinal center B-cell lymphomas were seen. These results suggest that Burkitt-like lymphoma with 11q aberration is a germinal center derived lymphoma closer to high grade B-cell lymphoma or diffuse large B-cell lymphoma rather than Burkitt lymphoma, although it shares some morphological features with Burkitt lymphoma. In work recently submitted for publication, we studied a series of cases of Histiocytic sarcoma. Histiocytic sarcoma is a rare malignant neoplasm that may occur de novo or in the context of a previous hematologic malignancy or mediastinal germ cell tumor. Here, we performed whole exome sequencing and RNA-Seq on twenty-one archival cases of primary histiocytic sarcoma. We identified a high number of genetic alterations within the RAS-RAF-MAPK pathway in 21/21 of cases, with alterations in NF1 (6/21), MAP2K1 (5/21), PTPN11 (4/21), BRAF (4/21), KRAS (4/21) and LZTR1 (1/21), including single cases with homozygous deletion of NF1, high-level amplification of PTPN11 and a novel TTYH3-BRAF fusion. Concurrent NF1 and PTPN11 mutations were present in 3/21 cases, and 5/7 cases with alterations in NF1 and/or PTPN11 had disease involving the gastrointestinal (GI) tract. Following unsupervised clustering of gene expression data, cases with NF1 and/or PTPN11 abnormalities formed a distinct tumor subgroup. A subset of NF1/PTPN11 wild-type cases had frequent mutations in B-cell lymphoma associated genes and/or clonal IG gene rearrangements. Our findings expand the current understanding of the molecular pathogenesis of this rare tumor and suggest the existence of a distinct subtype of primary histiocytic sarcoma characterized by NF1/PTPN11 alterations with predilection for the GI tract.